How Sensex and Nifty work.
One number, thirty or fifty companies, and what it tells you
The news said Sensex up 500. Your portfolio said otherwise.
Rajan watched the evening news and smiled. The anchor said markets had a great day. He opened his phone to check his own portfolio. Most of his stocks were down. He stared at the screen. The market had a great day. His holdings had a bad one. Both things were true at the same time.
An index is a single number that summarises what happened to a chosen basket of companies. Sensex tracks BSE's top 30. Nifty 50 tracks NSE's top 50. Two exchanges, two lists — one underlying idea. Any stock not in those lists can have its best day ever, and the Sensex won't move a point for it.
Both use free-float market cap weighting. Free-float means the shares actually available to the public — not the ones locked up with the promoter. A heavyweight company shifts the index far more than a small one. When Reliance has a good day, the Sensex feels it immediately. A smaller index member barely moves the needle.
Sensex started at 100 in 1978-79. It crossed 80,000 in July 2024 — roughly 800x in 45 years. That 80,000 is not a ceiling or a warning. It simply shows how far the basket has compounded from its starting point. The level tells you history. It says nothing about whether today's stocks are cheap or expensive.
A committee reviews both indices every six months. Companies that shrink or trade thinly get replaced by stronger ones. The basket always tracks the most relevant listed businesses in India at that moment.
The index is Mr. Market's daily mood
Imagine a neighbour called Mr. Market who shows up every morning with a price for all 50 of India's biggest listed companies combined. Some days he's cheerful and names high prices. Other days he panics and names low ones. The Nifty or Sensex is simply his mood, averaged across the basket. His number changes every second. The businesses behind it change far more slowly. Your job isn't to follow his mood — it's to understand what it's actually measuring.
Why this matters
If your equity mutual fund trails the Nifty 50 for five consecutive years, that deserves scrutiny. Either the fund manager is making poor choices, or fees are dragging your return below what a no-frills index fund would deliver. The index is not a goal in itself — it is the honest minimum bar. Any fund that cannot clear it over five years is quietly compounding underperformance against you. Knowing this one thing changes how you read every fund factsheet.
Move the slider. Watch one company shift the whole number.
Try the widget below. Pick a percentage move for a heavyweight Sensex company and see how many more index points it generates than the same move in a smaller one.
Same move, very different Sensex impact
Both stocks moved 5%. The heavyweight shifted Sensex by 400 points; the smaller stock shifted it by just 20 points. A 20× gap — not because one performed better, but because it carries 20× more weight in the index. This is free-float market cap weighting: the giants steer the number, not the percentage movers.
The index is your honest benchmark.
Where people go wrong
- Sensex up 500 means my stocks rose todayThe index tracks only 30 specific companies. Your holdings may not be in that basket and can move in the opposite direction on the same day.
- Sensex and Nifty are the same indexThey track different exchanges and different numbers of companies. Both tend to move together, but their compositions and daily moves are not identical.
- 80,000 sounds enormous — markets must be overvaluedThe Sensex started at 100 in 1978. 80,000 is cumulative compounding over 45 years, not a price tag. The raw level tells you nothing about today's valuation on its own.
- My fund beats FD returns, so it must be workingAn equity fund must be compared to an equity index, not a fixed deposit. If the Nifty consistently returns more than your fund year after year, those extra points are compounding against you.
Sensex tracks BSE's top 30 companies; Nifty 50 tracks NSE's top 50 — same idea, different exchanges.
Bigger companies have more weight. When Reliance rises, the Sensex feels it. A small member barely shifts the number.
80,000 is not a ceiling. It is 45 years of compounding from a starting point of 100.
When people see 80,000, they think the market has reached some ceiling. They forget it started at 100 decades ago. Every number since is just compounding doing its quiet work.